1. Field of the Invention
The present invention relates to a flat display screen. It more specifically applies to the assembly of two plates forming the bottom and the surface of the screen, and between which an internal spacing isolated from the outside is made.
2. Discussion of the Related Art
Conventionally, a flat screen is formed of two external generally rectangular spaced plates, for example made of glass. One plate forms the screen surface while the other forms the bottom of the screen generally provided with the emission means. These two plates are assembled by means of a sealing gasket. For a field effect display (FED) or a microtip display, or for a vacuum fluorescent display (VFD), vacuum is made in the space separating the two glass plates or this space contains a neutral atmosphere (rare gases).
FIG. 1 schematically shows in cross-sectional view the conventional structure of a microtip screen.
Such a microtip screen is essentially formed, on a first substrate 1, for example, made of glass, of a microtip cathode and of a grid. In FIG. 1, the cathode/grid assembly is designated with common reference 2. Cathode/grid 2 is placed opposite to a cathodoluminescent anode 3 made on a second transparent substrate 4, for example in glass, which forms the screen surface.
Cathode/grid 2 and anode 3 are made separately on the two substrates, or plates, 1 and 4, then assembled by means of a peripheral sealing gasket 5. An empty space 6 is formed between the two plates 1 and 4 to enable the circulation of the electrons emitted by the cathode towards anode 3.
The assembly of plates 1 and 4 is conventionally performed as follows.
First, spacers 7 for defining empty space 6 are glued onto cathode/grid 2. These spacers 7 are generally formed of glass balls regularly distributed so that space 6 between plates 1 and 4 is constant.
Then, cathode/grid 2 is submitted to a thermal vacuum processing having the purpose of degassing the cathode and evaporating the glue of spacers 7. A similar processing, not necessarily performed in vacuum conditions, is applied to anode 3.
According to a first conventional assembly method, a pumping tube 8 is provided on the free surface of a first plate 1. This tube 8 is, for example, in glass, and is sealed, by one of its open ends, above a hole made in plate 1 to establish a communication with space 6. According to this first method, tube 8 will be used in particular to create vacuum in space 6. Tube 8 is placed at the corner of plate 1 outside its useful surface. Then, a seal 5, for example a fusible glass cord, is deposited on the circumference of plate 1 or 4. Both plates 1 and 4 are then assembled by being pressed against each other, cathode/grid 2 facing anode 3, and this assembly being submitted to a temperature enabling the melting of cord 5. The obtained structure is then submitted, via tube 8, to a hot pumping which has the function of degassing space 6. This degassing is necessary, in particular, because of the gases emitted by fusible glass cord 5 during the sealing of the plates. Tube 8 is then closed at its free end after introducing therein a getter 9. The function of getter 9 is to absorb any contamination likely to appear during the subsequent operation of the screen. In FIG. 1, tube 8 has been shown as closed, that is, once the screen is completed.
A disadvantage of such an assembly method is that it is not adapted to a processing by screen batches. Indeed, although the step of sealing of the plates can be performed in a furnace containing several screens, the closing step is performed screen by screen.
To overcome this disadvantage, methods of vacuum assembly of a microtip screen which enable a batch processing have been provided.
FIG. 2 is a lateral view illustrating a conventional method of vacuum assembly of a microtip screen.
In such a process, plates 1 and 4 are maintained in vacuum conditions from their respective degassing thermal processings on and are placed against each other with an interposed fusible glass seal 5'. Peripheral fusible glass seal, or frame, 5' exhibits, before assembly, irregular surfaces of contact with plates 1 and 4. The whole is maintained under pressure, for example, by means of pliers 10. The whole as shown in FIG. 2 is then submitted to a temperature which enables the softening of frame 5' while remaining lower than its melting temperature. At such a softening temperature, the leaks 12 linked with the irregular surfaces of frame 5' are meant to enable an outlet of the gases emitted by the fusible glass frame inside the screen before the glass sealably assembles the two plates during its melting. This melting of frame 5' is obtained by bringing the whole to a temperature higher than the melting temperature of frame 5', which then causes, under the effect of the pliers, a crushing of frame 5' and the assembly of the screen, the distance between the plates being fixed by the spacers. As an alternative, the assembly formed of plates 1 and 4 and of frame 5' is progressively brought from the ambient temperature to the melting temperature, sufficiently slowly to enable the outlet of the gases emitted by leaks 12 before the sealed assembly.
In FIG. 2, the screen has been shown without any pumping tube. Such a pumping tube is no longer necessary in such an assembly method. The screen is however generally always associated with a tube (not shown) or with a glass box for housing a getter, not shown. This tube or box is also assembled by means of a fusible glass cord to one of the screen plates. The assembly of the tube or box is performed at the same time as the assembly of plates 1 and 4.
Although such a method is well adapted to a batch processing, it has the disadvantage of not sufficiently eliminating from inside the screen the gases emitted by cord 5'. Indeed, the fusible glass continues to degas when brought from its softening temperature to its melting temperature.
Another disadvantage of this assembly method is that the amount of degassing performed through leaks 12 is difficult to reproduce from one screen to another. Indeed, the surface irregularities of seal 5' are random and the flow rate of leaks 12 can thus not be controlled or reproduced. In addition to the fact that the remaining of the gases emitted by cord 5' inside the screen is prejudicial to the lifetime of the manufactured screens, this lifetime very significantly varies from one screen to another, though submitted to the same assembly process.